In their phylogenetic analysis using the cytochrome b gene, Nascimento et al. (2005) showed that populations of Alouatta caraya from Santa Cruz, Bolivia (Chaco) are differentiated from those in various localities in the state of Mato Grosso and (one specimen) Goiás further north. This indicates the possibility of two taxa of the Black Howler Monkey, rather than just one.

Justification:
This species is listed as Least Concern considering its large range, presence in several national parks, and ability to adapt to modified habitats. At present, it is not likely that the species, while declining, warrants listing in a threatened category under criterion A. Although its habitat is very fragmented, populations can live in relatively small areas and disturbed forest.

A wide-ranging species occurring thoughout much of the bush savanna (cerrado) and dry forests of central Brazil, south from middle and upper reach of the Amazonian tributaries, the rios Juruena, Xingu and Araguaia. It extends east of the Midde and upper Rio São Francisco in the west of the states of Minas Gerais and Bahia, occupying humid forest (known as brejos, resulting from orographic rainfall), semideciduous and deciduous (floresta de caatinga) and gallery forests. It extends into eastern Paraguay and in parts of the southern and central regions of the Paraguayan chaco, west of the Río Paraguai (Stallings 1985; Stallings et al,1989). Brown and Zunino (1994) descirbed its range in north-eastern Argentina, occurring in eastern Formosa and Chaco, the extreme north-eastern section of Santa Fe, northern Corrientes and the southern part of Misiones. Alouatta guariba occurs in the Atlantic forest remnants of northern Misiones. In Bolivia, this species is found exclusively east of the Rio Beni and is absent from true Amazonian forests of northern Beni and Pando. It is patchily distributed in the rest of lowland tropical Bolivia (R.B. Wallace pers. comm. 2007). Wallace et al. (2000) recorded it from Pajaral, east of the Rio Blanco, along with A. sara. A. sara they found in low densities (0.1 groups encountered per 10 km) in floodplain forest, whereas A. caraya was restricted to semideciduous forest patches.

Aguiar et al. (2007) recorded sympatry and probable hybridization with A. guariba in riparian forest along the left margin of the Rio Paraná (between Porto Figueira and Port Camargo) in the Ilhas e Várzeas do Rio Paraná Environmental Protection Area on the state of Paraná. This region is considered to be an ecotone between Cerrado (the typical domain of A. caraya) and the Atlantic forest (A. guariba). In São Paulo, it is restricted to the right (west) bank of the Rio Paraná above the mouth of the Rio Paranaiba.

Villalba et al. (1995) reported on its probable presence in the past, and possible existence still, in the extreme north-west of Uruguay.

Although widespread, Alouatta caraya is patchily distributed, and densities vary widely. In Argentina, Brown and Zunino (1994) recorded high densities in a number of sites: Chaco forest, Formosa 111 individuals/km²; Chaco forest, Corrientes 90 individuals/km²; gallery forest, 63 individuals/km²; and inundated forest 283 individuals/km². Arditi and Placci (1990) carried out surveys in gallery forests of the Chaco in Argentina (Riacho Pilagá, Estancia Guaycolec) and found lower numbers than those reported by Brown and Zunino (1994) resulting from surveys in 1980s: 11.7 individuals/km². Dvoskin et al. (2004) repeated the surveys there in 2001 and found that numbers had increased to 26 individuals/km². The highest densities occur in flooded forests and there have been numerous surveys of Alouatta caraya in this forest type (Pope 1968; Thorington Jr et al. 1994; Rumiz 1990; Zunino et al. 1996, 2001). Codenotti et al. (2002; see also Codenotti and Silva 2004) conducted a state-wide survey of remnant A. caraya populations in Rio Grande do Sul, Brazil. Populations were found to be small, isolated, mostly single groups, and densities were consistently low. Of 13 localities, only three had densities above 2 individuals/km² (Lageado do Celso 6.5 individuals/km²; a location in the municipality of Santiago 4.0 individuals/km²; and an urban park in the municipality of São Francisco de Assis 2.3 individuals/km²).

Alouatta caraya is found in semi-deciduous forests. Today, it occupies forest patches throughout a large part of the Cerrado of central Brazil, and the Pantanal of Mato Grosso. It occupies deciduous forest (caatinga alta) in some areas in southern Piaui, western Bahia and north-western Minas Gerais. In Argentina, it specializes in semi-dry deciduous forests and gallery forests as well as the typical island forests of the Chaco (Arditi and Placci 1990; Brown and Zunino 1994). In Bolivia, its found in the Chaco and Chiquitano forests, and gallery forest and forest islands in the Beni. They can be found in secondary forest and can adapt to disturbed and degraded habitats. In Paraguay, Stallings (1995; Stallings et al. 1989) reported that A. caraya occurs throughout the eastern forests and gallery forests of the southern and western Chaco along the ríos Paraguai and Pilcomayo, and the numerous small rivers that drain the chacoan plain. It can also be found in sporadic xeric forests, continuous with gallery forests. In Rio Grande do Sul, A. caraya occurs in mixed evergreen forest (Araucaria), forest patches in savanna, and seasonal deciduous forest (Codenotti et al. 2002; Codenotti and Silva 2004).

The howler monkeys are the large leaf-eaters of the South American primate communities. The molar teeth are particularly adapted for their chewing leaves through shearing. They spend up to 70% of their day lying and sitting about quietly among the branches, fermenting leaves in their enlarged caecums. Like the spider monkeys, they are prehensile-tailed, with a naked patch of skin on the under surface at the tip. Their most characteristic feature is the deep jaw which surround the enlarged larynx and hyoid apparatus, a resonating chamber. It is with this enlarged and highly specialized voice box that they produce their howls (grunts, roars and barks). Howling sessions, usually involving the entire group, can be heard particularly in the early morning and are audible at distances of 1-2 kms (Drubbel and Gautier 1993).

Howlers are the only New World primates that regularly include mature leaves in their diet, although softer, less fibrous, young leaves are preferred when they are available. Their folivory and ability to eat mature leaves is undoubtedly one of the keys to their wide distribution and the wide variety of vegetation types they inhabit. Mature fruit is the other important food item, especially wild figs (Ficus) in many regions, but they also eat leaf petioles, buds, flowers (sometimes seasonally very important), seeds, moss, stems and twigs, and termitaria.

Behavioural ecological field studies have been carried out by Zunino (1986) and collaborators (Zunino et al. 1996, 2001) in Argentina and Bicca-Marques (1994; 2003; Bicca-Marques and Calegaro-Marques 1994) in Brazil (see also review by Neville et al. 1988).

This species is threatened by habitat loss due to agricultural development for soy and cattle ranching in the Brazilian cerrado, soy in the Bolivian Chiquitano, and small-scale farms and cattle ranching in Argentina. Some subsistence hunting occurs across its range.